The invention relates to a radio FM receiver provided with a receiver portion for converting received radio frequency signals into center frequency quadrature signals, a center frequency filter circuit connected thereto for suppressing undesirable image frequencies, and a quadrature PLL (phase locked loop) demodulator with a VCO (voltage-controlled oscillator) for demodulating the obtained center frequency quadrature signals, said PLL being attuned to the carrier frequency within the selected frequency band.
Amplitude and phase errors occur in the receiver portion and the filter circuit, with the result that undesirable image frequencies are insufficiently suppressed. These amplitude and phase errors are dependent on the input frequency, i.e. on the carrier frequency within the selected frequency band and on the amplitude of the received input signal. It is known to correct such amplitude and phase errors between the quadrature channels in the receiver, for example from U.S. Pat. Nos. 5,134,404 or 5,263,196. The correction methods described therein, however, are highly complicated and insufficiently accurate.
It is an object of the invention to provide means whereby in a comparatively simple manner the amplitude and phase errors are eliminated to a high degree independently of the input frequency and the amplitude of the input signals, such that a considerably improved image frequency suppression is obtained.
According to the invention, the radio FM receiver is for this purpose characterized in that an error detector is present for detecting amplitude and phase errors in the quadrature channels, which error detector, in response to the quadrature components of the center frequency signals, the quadrature signals of the VCO in the quadrature PLL demodulator, and the filtered difference of said quadrature components of the center frequency signals, supplies amplitude and phase correction signals to a quadrature channel correction network which is provided between the filter circuit and the quadrature PLL demodulator.
In a preferred embodiment, the error detector is provided with a second quadrature PLL demodulator with a VCO (voltage-controlled oscillator) for demodulating the obtained center frequency quadrature signals, wherein the PLL is attuned to the carrier frequency of the image signals within the selected frequency band, with a non-linear processing unit which generates mixing signals in the first and second PLL demodulators after being supplied with the quadrature signals from the two VCOs, and with two mixing stages to each of which one of the mixing signals is supplied as well as the filtered difference of the quadrature signals from the first quadrature PLL demodulator, which mixing stages give off said correction signals after filtering.
It is furthermore possible to correct amplitude and phase errors in the quadrature channels in that only a calibration signal is supplied to the input of the receiver portion instead of reception signals obtained through the antenna. The error detector can be connected during such a calibration, whereas the connection of the error detector to the quadrature channels may be broken outside this calibration. Although the error detector can thus be given a simpler construction, the amplitude and phase errors in the quadrature channels are only corrected then for a certain calibration frequency and a certain calibration signal amplitude. Amplitude and phase errors will continue to arise for frequencies and amplitudes differing from the above.